This invention pertains to microlithography (projection-transfer of a pattern, defined on a reticle, to a suitable substrate). Microlithography is a key technology used in the manufacture of microelectronic devices such as integrated circuits, displays, and the like. More specifically, the invention pertains to microlithography performed using a charged particle beam such as an electron beam or ion beam. Even more specifically, the invention pertains to calibrations, as performed with a charged-particle-beam microlithography apparatus, directed to achieving accurate alignment of the reticle with the substrate.
In charged-particle-beam (CPB) microlithography (i.e., microlithography performed using a charged particle beam such as an electron beam or ion beam), as in optical microlithography (i.e., microlithography performed using visible or ultraviolet light), obtaining accurate alignment between the reticle and the substrate is extremely important. Current microlithography apparatus include sophisticated devices for determining reticle-substrate alignment. In a CPB microlithography apparatus as currently available, these alignments involve impinging a charged particle beam on a mark on the substrate or substrate stage and detecting charged particles (e.g., electrons) backscattered from the mark.
In various techniques for determining reticle-substrate alignment using a charged particle beam, it is important to be able to determine and calibrate the angle of incidence of the beam on the substrate (specimen) surface. A conventional method for calibrating the angle of incidence of an electron beam on a specimen surface is shown in FIG. 3. In the figure, an electron beam EB propagating along an axis AX is deflected by a deflector 70 to impinge on a surface 100 of a specimen (e.g., semiconductor wafer). The surface 100 includes an alignment mark 101. When electrons of the beam EB impinge on the alignment mark 101, backscattered electrons are produced that propagate to and are detected by a backscattered-electron (BSE) detector 72 situated upstream of the surface 100. Typically, the deflector 70 deflects the beam EB in a scanning manner over the mark 101.
To perform the conventional calibration method, a calibration specimen (having a surface 100 bearing an alignment mark 101) is mounted on a specimen stage. The specimen stage is positioned such that the surface 100 is at a desired first axial position (first xe2x80x9cheightxe2x80x9d). With the specimen positioned in this manner, the electron beam EB irradiates and is scanned across the specimen. The position of the alignment mark 101 is determined from a BSE signal waveform produced by the BSE detector 72, based on backscattered electrons propagating from the alignment mark 101.
After obtaining the BSE signal waveform from the alignment mark 101, the specimen table is moved axially (in the Z-direction) to place the surface 100 at a second xe2x80x9cheightxe2x80x9d (the surface at the second height is denoted 100xe2x80x2, and the alignment mark on the surface at the second height is denoted 101xe2x80x2). The difference between the first height and second height is denoted xcex94Z. With the specimen positioned at the second height, the electron beam EB irradiates and is scanned across the specimen. The position of the alignment mark 101xe2x80x2 is determined in the same manner as described above with respect to the alignment mark 101.
Referring further to FIG. 3, a respective angle xcex8 at which the electron beam EB is incident to the specimen surface 100, 100xe2x80x2 is found by determining xcex94X (difference in X-axis position of the alignment mark 101xe2x80x2 relative to the X-axis position of the alignment mark 101) and xcex94Z (change in elevation of the specimen stage). Normally, whenever the deflection angle of the electron beam relative to the axis AX is zero degrees, the angle of incidence xcex8 of the electron beam on the specimen surface should be zero degrees. Consequently, an electron-optical system (through which the electron beam passes) or the specimen stage is adjusted such that the angle xcex8 is zero degrees.
In the conventional method described above, the accuracy with which the angle xcex8 is determined is a function of the magnitude of Z-direction movement (xcex94Z) of the specimen stage and the accuracy with which the alignment mark is detected. For example, if the Z-direction movement xcex94Z is 10 xcexcm and the alignment-mark detection accuracy is 0.1 xcexcm, then the approximate accuracy (xcex94xcex8) with which the angle xcex8 can be detected is determined as follows:
xcex94xcex8=arctan(0.1 xcexcm/10 xcexcm)=10 mrad
(10 mrad is a common value for this accuracy). If the Z-direction movement (xcex94Z) is 100 xcexcm and the alignment-mark detection accuracy is 10 nm, then the accuracy with which the angle xcex8 should be detected is approximately 100 xcexcrad. To obtain this accuracy, an angle-detection accuracy of approximately 10 xcexcrad is required. Unfortunately, with a conventional electron-beam microlithography apparatus, such accuracy is difficult to achieve.
Generally speaking, it is difficult to improve the movement in the Z-direction and the alignment-mark detection accuracy more than the respective amounts discussed above. It also is difficult to obtain an alignment-mark detection accuracy of less than 100 xcexcrad. In addition, whenever the height of the specimen stage is changed, accompanying lateral shifts also are encountered frequently. Hence, conventional methods for performing reticle-substrate alignment have factors that contribute significantly to degradations in detection accuracy.
In view of the shortcomings of the prior art as summarized above, an object of the present invention is to provide, inter alia, reticle-substrate calibration methods (as used with a charged-particle-beam (CPB) microlithography apparatus) that achieve high-accuracy calibration results using a simple procedure. Another object of the present invention is to provide microelectronic-device manufacturing methods by which high-accuracy patterns can be formed using a CPB microlithography apparatus calibrated according to the invention.
To such ends, and according to a first aspect of the invention, methods are provided (in the context of performing a CPB microlithography of a specimen using a CPB microlithography apparatus) for calibrating the CPB microlithography apparatus. In a representative embodiment of such a method, a specimen is provided presenting a surface having a crystalline structure. A charged particle beam (e.g., electron beam) is irradiated onto an area of the surface by scanning the beam in X- and Y-dimensions. While monitoring X- and Y-dimension beam-scanning coordinates of the scanned area, backscattered charged particles produced by the area being irradiated are detected. A corresponding backscattered-particle electrical signal is produced. The signal contains signal-amplitude data as a function of the X- and Y-dimension beam-scanning coordinates. The data in the signal are processed to produce a map pattern of signal amplitude as a function of X- and Y-dimension beam-scanning coordinates. Also produced are data regarding whether a center of the map pattern is aligned with an origin of X- and Y-dimension beam-scanning axes. If the center of the map pattern is not aligned with the origin of X- and Y-dimension beam-scanning axes, then an adjustment is performed to achieve such alignment so as to calibrate the apparatus.
In the foregoing method, the data-processing step can include calibrating an angle of incidence of the charged particle beam on the surface. Also, the step of providing a specimen can include providing a monocrystalline silicon specimen of which the presented surface has a 111 crystal-lattice structure.
In another representative embodiment, a specimen (presenting a surface having a given crystal-orientation plane) is mounted on a specimen table of the CPB microlithography apparatus. A charged particle beam (e.g., electron beam) is passed through a CPB-optical system so as to cause the beam to be incident on an area of the surface. A deflector is provided, and the deflector is supplied with an electrical signal serving to deflect the charged particle beam laterally as the beam passes through the CPB-optical system. This causes the charged particle beam incident on the area of the surface to move in a scanning manner over the area in X- and Y-dimensions. The X- and Y-dimension beam-scanning coordinates of the scanned area are monitored. Meanwhile, backscattered electrons (BSE) produced by the area being irradiated are detected to produce data concerning, for the detected backscattered electrons, BSE signal amplitude as a function of X- and Y-dimension beam-scanning coordinates. From the processed data, a relationship between BSE signal amplitude and the electrical signal provided to the deflector is determined. From this relationship, an angle of incidence of the charged particle beam on a specific locus in the area of the surface is determined. In this method, if the angle of incidence is not a pre-specified value, then an adjustment is performed to achieve the pre-specified value.
According to another aspect of the invention, CPB microlithography apparatus are provided. An embodiment of such an apparatus comprises a specimen stage and a CPB source. The specimen is mounted on the specimen stage, wherein the specimen desirably presents a surface having a crystal-lattice structure. A CPB source is situated and configured to produce a charged particle beam propagating downstream of the source. A CPB-optical system is situated and configured to irradiate the charged particle beam, from the CPB source, onto an area of the surface while scanning the beam relative to the specimen over the area. A backscattered-particle detector is situated and configured to detect backscattered charged particles propagating from the irradiated region of the specimen and to produce, from the detected particles, a corresponding backscattered-particle signal. The backscattered-particle signal exhibits a characteristic corresponding to a property of the crystal-lattice structure of the surface. The apparatus also includes a calibration controller connected to the backscattered-particle detector and configured to receive the backscattered-particle signal and to perform a calibration of the CPB microlithography apparatus based on a value of a parameter of the backscattered-particle signal. Desirably, the characteristic of the backscattered-particle signal is signal amplitude as a function of X- and Y-direction beam-scanning coordinates. The calibration controller desirably is configured to produce a map pattern of backscattered-particle signal amplitude as a function of X- and Y-dimension beam-scanning coordinates. The calibration controller can be configured to obtain data regarding whether a center of the map pattern is aligned with an origin of X- and Y-dimension beam-scanning axes.
As noted above, the specimen used at least for calibration desirably is made of a material having crystalline properties (e.g., monocrystalline silicon). A charged particle beam irradiates and scans an area of the specimen. The angle at which the beam is incident to the specimen surface is detected by processing a signal waveform of a characteristic BSE signal from the specimen surface. For example, backscattered charged particles from a crystalline surface produce a signal in which a change in amplitude of the signal is as function of the angle of incidence of the beam on the specimen surface and of the specific location (relative to the crystal lattice) being impinged by the beam. This phenomenon arises because of differences in how easily an incident charged particle beam can pass through the specimen as a result of the crystalline properties of the specimen.
Whenever the incidence angle of the charged particle beam on the specimen surface exhibits such changes, the position at which the backscattered charged particle (originating from the irradiated crystalline surface) is detected also moves according to characteristics of the crystalline surface. As a result, the beam axis through the CPB-optical system can be detected from a relationship between the detection position of the waveform and the angle of incidence of the beam.
Using methods according to the invention, it is possible to obtain a highly accurate detection of beam position compared to conventional methods. In addition, methods according to the invention eliminate the need to change the height of the specimen during calibration. Hence, factors related to transverse shifting of the specimen following changes in the height of the specimen can be eliminated.
The foregoing and additional features and advantages of the invention will be more readily apparent from the following detailed description, which proceeds with reference to the accompanying drawings.
The present invention can be applied to any type of CPB microlithography apparatus or method, including (but not limited to) block exposure, whole-reticle transfer, divided-reticle transfer, and direct drawing.